Cathode-ray tube provided with at least one electron gun for producing a number of individually prefocused electron beams

ABSTRACT

A cathode-ray tube provided with an electron gun system for producing two or more electron beams that are individually prefocused by two common, substantially circular-cylindrical electrodes. The beams are in a common plane so that the distance from the beams to the axis of the circular-cylindrical electrodes is not the same for all of the beams. The beams are individually prefocused and this prefocusing diminishes as the distance from the beam to the axis of the circular electrodes increases. A common, plate-shaped grid serves as a prefocusing electrode having apertures which increase in size as the distance therefrom to the axis of the circular-cylindrical electrode increases. Alternatively, the spacing between the prefocusing electrode and an adjacent electrode is made greater as the distance of the axis of the beam to the axis of the cylindrical electrode increases.

United States Patent [72] Inventor Piet Gerard Joseph Barten Emmasingel, Eindhoven, Netherlands [21] Appl. No. 8,286 [22] Filed Feb. 3, 1970 [45] Patented Oct. 5, 1971 [32] Priority Feb. 8, 1969 [3 3] Netherlands [31 6902024 [54] CATHODE-RAY TUBE PROVIDED WITH AT LEAST ONE ELECTRON GUN FOR PRODUCING A NUMBER OF INDIVIDUALLY PREFOCUSED ELECTRON BEAMS 8 Claims, 3 Drawing Figs.

[52] US. Cl 313/85, 313/70 C, 313/92 B, 315/13 CG [51] Int. Cl H01j 29/46 [50] Field of Search .1 315/13 CG; 313/70 C, 82, 85

[56] References Cited UNITED STATES PATENTS 2,863,091 12/1958 Epstein et al. 315/13 CG Primary Examiner-Rodney D. Bennett, Jr. Assistant Examiner-Brian C. Ribando Attorney-Frank R. Trifari ABSTRACT: A cathode-ray tube provided with an electron gun system for producing two or more electron beams that are individually prefocused by two common, substantially circular-cylindrical electrodes. The beams are in a common plane so that the distance from the beams to the axis of the circularcylindrical electrodes is not the same for all of the beams. The beams are individually prefocused and this prefocusing diminishes as the distance from the beam to the axis of the circular electrodes increases. A common, plate-shaped grid serves as a prefocusing electrode having apertures which increase in size as the distance therefrom to the axis of the circular-cylindrical electrode increases. Alternatively, the spacing between the prefocusing electrode and an adjacent electrode is made greater as the distance of the axis of the beam to the axis of the cylindrical electrode increases.

CATHODE-RAY TUBE PROVIDED WITH AT LEAST ONE ELECTRON GUN FOR PRODUCING A NUMBER OF INDIVIDUALLY PREFOCUSED ELECTRON BEAMS The invention relates to a cathode-ray tube provided with an electron gun system for producing two or more electron beams which are individually prefocused, said system comprising at least two common, substantially circular-cylindrical and substantially coaxial electrodes. The invention relates in particular to such a cathode-ray tube provided with a display screen having a number of differently luminescent substances and a color selection electrode arranged at a short distance from the display screen, a number of electron beams corresponding to the number of luminescent substances being produced by the electron gun system, said beams being caused to converge in the plane of the color selection electrode by means of a lens field obtained by means of the common substantially circular-cylindrical electrodes.

The electron beams in the color tube are caused to converge substantially in the plane of the color selection electrode, for example, a shadow mask electrode, by means of the common substantially circular-cylindrical electrodes. This lens effect can be obtained by means of one substantially circular-cylindrical grid and a conductive coating on the envelope, by means of two substantially circular-cylindrical grids which, in a special case, have substantially equal diameters, or by means of more than two substantially circular-cylindrical grids. Such an electrostatic convergence system is used in particular when the beams are situated close together because in this case there is little space for the pole shoes required in the magnetic convergence. This may be the case, for example, in a cathode-ray tube in which the various electron beams are produced by one gun. Each of the electron beams is furthermore focused on the display screen or at least in a plane situated near such screen. for example, that of the color selection electrode. This is effected partly by the lens effect between successive grids of the gun or of the guns which thus produce a prefocusing, and partly by the lens effect of the convergence system which thus produces a postfocusing.

The postfocusing of each beam by the converging lens is the same for each beam if the axes of the beams are situated at substantially equal distance from the axis of the circular-cylindrical electrodes. This is the case, for example, when in a cathode-ray tube three electron beams are produced, the centers of which form the corners of an equilateral triangle. However, when the beams are deflected, the convergence is not maintained and dynamic convergence should be used. In principle it is possible, however, to converge the electron beams over the whole screen without dynamic convergence means if the axes of the beams are situated substantially in one plane and a given type of deflection coil is used. The axes of the beams are then no longer situated at substantially equal distance from the axis of the circular-cylindrical electrodes. However, then the problem presents itself that as a result of the fact that beams are situated at a different distance from the axis of the circulancylindrical electrodes, the postfocusing for the beams is not the same and actually the postfocusing is then stronger as a result of the edge fields of the converging lens, according to the axis of the beam within the circularcylindrical electrodes is situated farther from the axis of said electrodes.

The invention is based on the recognition of the fact that, in order to prevent said drawback, the prefocusing of the beams should be different so that the overall focusing formed by the prefocusing and the postfocusing is substantially equal for each beam. According to the invention, the distance for at least one electron beam from its axis to the axis of the substantially circular-cylindrical electrodes is different from that distance for the other electron beams and the prefocusing of an electron beam is weaker according as said distance is larger.

If the electron beams are produced by various guns, the difference in prefocusing can be obtained by a difference in voltage of the electrodes which produce the prefocusing. This case presents itself, for example, when the various electron guns are constructed in the same manner structurally. However, it may be considered a drawback that the number of voltages to be supplied to the system of electron guns ought to be increased in this case. Therefore, a solution is preferred in which a different prefocusing is realized nevertheless while using the same voltages in the various electron guns. In this case, the structural construction of the electrodes which produce the prefocusing is different. In the case of one gun for producing the various beams in which the electrodes, except for one electrode, are common for the beams it is even not possible to give different voltages to the electrodes. In this case the cathode-ray tube is provided in particular with an electron gun for producing at least three electron beams, the axes of which are situated in a flat plane through the axis of the substantially circular-cylindrical electrodes.

The invention relates in particular to a cathode-ray tube which is provided with an electron gun for producing a number of electron beams in which the electron gun comprises a separate cathode for each electron beam, furthermore a common first grid which is provided with an aperture for each beam, a common second grid which serves as an accelerating electrode and is provided with an aperture for each beam, a common third grid which is provided with an aperture for beam beam, and a common, substantially circular-cylindrical fourth grid, in which in each grid the distance from the centers of the apertures to the axis of the fourth grid are not the same for all the beams and the apertures in the third grid are larger according as the distance from the center of the aperture to the axis of the fourth grid is larger. Due to the difference in size of the apertures in the third grid, the difference in the prefocusing is obtained.

The invention furthermore relates in particular to a cathode-ray tube which is provided with an electron gun for producing a number of electron beams in which the electron gun comprises a separate cathode for each electron beam, furthermore a common first grid which is provided with an aperture for each beam, a common second grid which serves as an accelerating electrode and is provided with an aperture for each beam, a common third grid which is provided with an aperture for each beam, and a common, substantially circularcylindrical fourth grid, in which in each grid the distances from the centers of the apertures to the axis of the fourth grid are not the same for all the beams and the distance between the second and the third grid at the area of an aperture is larger according as the distance from the center of the aperture to the axis of the fourth grid is larger. Due to the difference in distance between the second and the third grid, the difference in the prefocusing is obtained.

In order that the invention may be readily carried into effect, it will now'be described in greater detail by way of example, with reference to the accompanying drawing, in which:

FIG. 1 is a cross-sectional view of cathode-ray tube,

FIG. 2 shows certain parts of the cross-sectional view of FIG. I on an enlarged scale, and

FIG. 3 shows another construction of certain parts of the cross-sectional view of FIG. I on an enlarged scale.

A cathode-ray tube 1 in FIG. 1 comprises a diagrammatically shown gun 2 which produces three electron beams the centers of which are situated in a plane through the axis of the gun, the axis of the central electron beam coinciding with the axis of the electron gun. The electron gun 2 converges said three electron beams or a shadow mask 3 after which they each impinge upon certain parts of a luminescent screen 4. The scanning of the screen is effected by a deflection device 5 shown diagrammatically.

FIG. 2 is a cross-sectional view of the part of the neck of the tube through the axis of the gun. The gun comprises three cathodes 6, 7 and 8, a common first grid 9 which is provided with apertures 10, l1 and 12 for the beams originating from the cathodes 6, 7 and 8, a common second grid 13 which is provided with apertures l4, l5 and 16, and a common third grid 17 which is provided with apertures 18, 19 and 20, the

aperture 19 being smaller than the apertures 18 and 20. The centers of the apertures 11, and 19 are situated on the axis 21 of the gun, the centers of the apertures 10, 14 and 18 being situated on a line 22 parallel to the axis 21 and the centers of the apertures 12, 16 and being situated on a line 23 also parallel to the axis 21. In this case the line 22 coincides with the axis of the cathode 6, the axis 21 coincides with the axis of the cathode 7, and the line 23 coincides with the axis of the cathode 8. The gun furthermore comprises a common fourth grid 24 and a common circular-cylindrical fifth grid 25. The fourth grid consists of two interconnected circular-cylindrical portions 26 and 27. The plate-shaped third grid 17 comprises an annular elevation 28 on the side facing the fourth grid. The postfocusing takes place in the converging lens which is an accelerating lens comprising the fourth grid 24 and the fifth grid 25, the lens field being produced between the circular-cylindrical portion 27 and the circular-cylindrical grid which have the same diameters.

in a certain case the distance from the cathodes 6, 7, 8 to the first grid 9 is 0.09 mm., from the first grid 9 to the second grid 13 it is l.0 mm. and from the second grid 13 to the third grid 17 it is 1.5 mm. The first grid 9 has a thickness of 0.20 mm., the second grid 13 has a thickness of 0.50 mm. and the third grid 17 also has a thickness of 0.50 mm. The circular apertures l0, l1 and 12 in the first grid 9 have a diameter of 0.75 mm., the circular apertures 14,15 and 16 in the second grid 13 have a diameter of 0.75 mm., the circular apertures 18 and 20 in the third grid 17 have a diameter of 2.0 mm., and the circular aperture 19 in the third grid 17 has a diameter of 1.5 mm. The distance from the line 22 through the centers of the apertures 10, 14 and 18 to the axis 21 of the gun is 3.5 mm. and the distance from the line 23 through the center of the apertures 12, 16 and 20 to the axis 21 of the gun is also 3.5 mm. The inside diameter of the portion 26 of the fourth grid 24 is 14 mm. and that of the portion 27 is 20 mm. The diameter of the annular elevation 28 is 14 mm., so that this is equal to the inside diameter of the portion 26 of the fourth grid 24 facing the third grid 17. The inside diameter of the fifth grid 25 is also 20 mm. The dimension of the elevation 28 in the direction of the axis of the gun is 1.5 mm., that of the portion 26 in said direction is 7 mm., that of the portion 27 in the said direction is l8 mm., and that of the fifth grid 25 in said direction is 10 mm. The distance between the elevation 28 and the portion 26 is 2 mm. and between the portion 27 and the fifth grid 15 it is also 2 mm. This gun can be operated at the following voltages:

cathode 0 volt and I volt first grid 0 volt second grid 1.700 volt third grid 350 volt fourth grid 4,300 volt fifth grid 25.000 volt Since the diameter of the aperture 19 is smaller than that of the aperture 18 and 20, it is achieved that the prefocusing of the central beam is stronger.

FIG. 3 is a cross-sectional view of the neck portion of the tube according to another construction taken through the axis of the gun. Similar components are referred to by the same reference numerals as in FIG. 2. The gun comprises a common second grid 30 which is provided with apertures 31, 32 and 33 and a common third grid 34 which is provided with apertures 35, 36 and 37 which have the same diameters. The surface of the second grid 30 which is situated on the side of the first grid is flat. The other surface of the grid 30 is such that the thickness at the area of the aperture 32 is larger than at the area of the apertures 31 and 33. As a result of this the distance between the second grid 30 and the third grid 34 at the area of the central beam is smaller than at the area of the other beam, as a result of which it is achieved that the prefocusing of the central beam is stronger than of the other beam.

What is claimed is:

1. A cathode-ray tube comprising a hermetically sealed envelope, an electron gun system within said envelope and comprising a plurality of cathodes for producing a plurality of electron beams in a plane, a postfocusing cylindrical electrode system for said beams, said beams projecting in the direction of the axis of said cylindrical electrode system and being spaced at difierent distances from said axis, beam prefocusin g means individual to said beams and interposed between said cathodes and said postfocusing means, said prefocusing means having a greater focusing action for electron beams closer to said axis than for beams located at greater distances from said axis, and target means for receiving said electron beams.

2. A cathode-ray tube as claimed in claim 1 wherein said beam prefocusing means comprises first, second and third grids respectively said grids having an aperture for each beam, the size of the apertures in the third grid determining the differences in said focusing action.

3. A cathode-ray tube as claimed in claim 2 wherein the size of the apertures in the third grid is determined by the distances of said electron beams from the axis of said postfocusing cylindrical electrode system.

4. A cathode-ray tube as claimed in claim 2 wherein the size of the apertures in the third grid are larger than the apertures in the first and second grids.

5. A cathode-ray tube as claimed in claim 2 wherein the distances between the second and third grids determines the differences in said focusing action.

6. A cathode-ray tube as claimed in claim 5 wherein the distance between the second and third grids is determined by the distances of said electron beams from the axis of said postfocusing cylindrical electrode system.

7. A cathode-ray tube as claimed in claim 1 wherein said target means comprises a color selection electrode spaced at a distance from said postfocusing cylindrical electrode system and a screen essentially consisting of a plurality of different luminescent substances, the number of electron beams corresponding to the number of luminescent substances being produced by said electron gun system.

8. A cathode-ray tube as claimed in claim 1 wherein said electron gun system produces at least three electron beams, the axes of which are situated in a flat plane through the axis of the postfocusing cylindrical electrode system. 

1. A cathode-ray tube comprising a hermetically sealed envelope, an electron gun system within said envelope and comprising a plurality of cathodes for producing a plurality of electron beams in a plane, a postfocusing cylindrical electrode system for said beams, said beams projecting in the direction of the axis of said cylindrical electrode system and being spaced at different distances from said axis, beam prefocusing means individual to said beams and interposed between said cathodes and said postfocusing means, said prefocusing means having a greater focusing action for electron beams closer to said axis than for beams located at greater distances from said axis, and target means for receiving said electron beams.
 2. A cathode-ray tube as claimed in claim 1 wherein said beam prefocusing means comprises first, second and third grids respectively said grids having an aperture for each beam, the size of the apertures in the third grid determining the differences in said focusing action.
 3. A cathode-ray tube as claimed in claim 2 wherein the size of the apertures in the third grid is determined by the distances of said electron beams from the axis of said postfocusing cylindrical electrode system.
 4. A cathodE-ray tube as claimed in claim 2 wherein the size of the apertures in the third grid are larger than the apertures in the first and second grids.
 5. A cathode-ray tube as claimed in claim 2 wherein the distances between the second and third grids determines the differences in said focusing action.
 6. A cathode-ray tube as claimed in claim 5 wherein the distance between the second and third grids is determined by the distances of said electron beams from the axis of said postfocusing cylindrical electrode system.
 7. A cathode-ray tube as claimed in claim 1 wherein said target means comprises a color selection electrode spaced at a distance from said postfocusing cylindrical electrode system and a screen essentially consisting of a plurality of different luminescent substances, the number of electron beams corresponding to the number of luminescent substances being produced by said electron gun system.
 8. A cathode-ray tube as claimed in claim 1 wherein said electron gun system produces at least three electron beams, the axes of which are situated in a flat plane through the axis of the postfocusing cylindrical electrode system. 